The present invention relates to a device for forcing a piston ring radially outwardly and more particularly a device for forcing a piston ring radially outwardly adapted for use in a reciprocating compressor or with a piston valve which reciprocates in a cylinder under a gas pressure or an air cylinder driving valve.
Generally, an oilless reciprocating compressor has a piston ring for compressing a gas. The piston ring is forced against the bore wall of a cylinder so as to minimize the leakage of the gas being compressed. Various methods have been devised and demonstrated for forcing a piston ring against the bore wall of a cylinder; one of them uses a plunger as shown in FIG. 3.
In FIG. 3, reference numeral 21 designates a cylinder; 22, a piston; 23, a circumferential groove of the piston 22; 24, a piston ring loosely fitted into the groove 23; 25, a radial hole of the piston ring 25; 26, a sleeve made from Teflon (trademark) and securely fitted into the hole 25; 27, a plunger rod in the form of a hollow cylinder and loosely fitted into the sleeve 26; 28, a plunger securely attached to the radially outward end of the plunger rod 27 and adapted to force the piston ring to the bore wall of the cylinder 21; 29, a coiled spring disposed in compressed state within the plunger rod 27; 30, a guide pin for the coiled spring 29; and 31, spring shoes or cushions.
In the device as shown in FIG. 3, the compressed spring 29 always forces the plunger to move in the radially outward direction (right in FIG. 3) so that the piston ring 24 is always made into intimate contact with the bore wall of the cylinder 21.
When the piston ring 24 is new and is not worn out, the coiled spring 29 is bent because of large deflection or compression received and is made into contact with the inner wall of the plunger rod 27. As the compression and suction cycles are repeated, the plunger 28 is caused to slightly vibrate in the direction in which the coiled spring 29 pushes the plunger 28 so that friction is produced between the coiled spring 29 and the inner wall of the plunger rod 27, resulting in wear or damage of the former and/or the latter. This causes lack of force for forcing the piston ring 24 to the bore wall of the cylinder 21 and consequently the piston ring 24 or the piston 22 itself is damaged.
When the spring shoes 31 at the ends of the coiled spring are inclined or deformed, the coiled spring 29 is bent so that the above-described problem also arises.
If the piston ring 24, which is always forced into intimate contact with the bore wall of the cylinder 21, is made of a material which tends to be broken (for instance, carbon) and if a portion of the piston ring 24 against which is forced the plunger 28 is lost because of wear or damage, the plunger 28 is forced to move out of the groove 23 and in some cases the plunger rod 27 as well as the coiled spring 25 are also moved out of the hole 25. As a result, the plunger 28 is forced into direct contact with the bore wall of the cylinder 21 and is reciprocated in unison with the piston 22. Thus, the cylinder bore and/or the plunger 28 is damaged and the piston 22 is also damaged.
Therefore there is a problem that in order to prevent the above described damages, inspection and replacement of component parts must be made at a relatively short frequency.